The number of permutations of length n in a particular geometric grid class.

Terms

a(0) =1a(1) =2a(2) =6a(3) =19a(4) =56a(5) =157a(6) =428a(7) =1149a(8) =3058a(9) =8097a(10) =21370a(11) =56279a(12) =147990a(13) =388727a(14) =1020252a(15) =2676139a(16) =7016372a(17) =18389377a(18) =48184544a(19) =126229809a(20) =330635974a(21) =865940277a(22) =2267709166a(23) =5938235819a(24) =15549095466a(25) =40713244907a(26) =106599027888a(27) =279100615999a(28) =730736374568